Entry into middle school is a critical transition; children are faced with increased academic expectations, have to renegotiate relationships with their parents, and become enmeshed in the peer realm. Peers have an increasing influence on child adaptive and maladaptive behavior during middle school, and parents must acquire new skills to promote child adjustment. Some children fail to navigate the transition to middle school successfully, and engage in socially deviant behaviors including shoplifting, truancy, precocious sexual behavior, and alcohol or drug use; other children develop problems with depression or anxiety. These behavioral outcomes have a high public health impact, both in terms of the costs to child-focused service systems, and to the long-term health and well-being of children and families. Although it is well known that parents and peers have a substantial influence on children during this developmental period, few studies have examined the impact of children's genetically-influenced characteristics on these important relationships (evocative gene-environment correlation, rGE). Even fewer studies have examined how genetic vulnerabilities moderate peer or parenting effects during this period (gene-environment interaction, GxE). The adoption design is one of the strongest tools for detecting evocative rGE and GxE because it can disaggregate the influences of genetic propensities from family environmental effects. We can separate genetic and postnatal environmental influences because the former comes from the biological parents, and the latter from the adoptive family, and we can then examine how they work together across development. The proposed study provides a unique opportunity to study GxE and evocative rGE as they unfold from infancy to early adolescence. Building on a study of 361 children who were adopted at birth, their adoptive parents, and their biological parents, we propose to collect new data at child age 11 and 13 years in order to span the developmental transition from elementary school to middle school. Participants have previously been assessed in-person at regular intervals from infancy to age 8. In the proposed study, adopted children and their adoptive parents will be assessed at approximately age 11 (end of elementary school) and 13 (two years later, in middle school), with a peer assessment at age 13 and a biological parent assessment at child age 11. Innovations of the proposed study include the unique ability of the adoption design to disentangle the effects of genetic and environmental influences and study their interplay; the rich existing data, including a wealth of data on fathers and the focus on mechanisms underlying G-E interplay. Specific aims are: (1) To identify specific family and peer processes that combine with genetic influences to promote social competence and prevent problem behavior through child age 13 (GxE); (2) To identify genetically-influenced child behaviors that evoke specific parenting and peer processes (rGE); and (3) To examine HPA axis regulation (diurnal rhythm) as a biological mechanism that may moderate parent and peer influences and mediate GxE effects on child adjustment. By furthering the understanding of G-E interplay, and the biological mechanisms that underlie environmental influences on development, findings from this study could be used to identify children at heightened risk for maladjustment during this critical transition, and factors that promote social competence and well-being.